(1) Field of the Invention
The present disclosure relates to a power supply device and an image forming device. In particular, the present disclosure relates to a technology for guaranteeing low electric power loss and accurate supply of voltages when load size is increased.
(2) Related Art
A typical power supply device used in an image forming device is capable of outputting multiple voltages by including multiple transformers and multiple control units each provided to control a different one of the transformers. In Japan, commercial power sources output 100 V AC power. For example, a power supply device receiving input of such power and outputting two different DC voltages, namely a low voltage (e.g., 5 V) and a high voltage (e.g., 24 V), is provided with a combination of a control circuit and a transformer for each voltage. In such structure, each combination of a control circuit and a transformer performs feedback control to output a voltage within a rated voltage range.
In view of such conventional technology, a proposal is being made of a power supply device that includes only one combination of a control circuit and a transformer that outputs both a low voltage and a high voltage, in order to reduce cost and device size. Refer to Japanese Patent Application Publication No.: H08-211790 for one example of such a power supply device. According to this technology, due to the necessity of controlling output low voltage with higher accuracy compared to output high voltage, the output low voltage is monitored for feedback control.
With such feedback control, the output high voltage may fluctuate considerably depending upon the amount of load current. In view of this, Japanese Patent Application Publication No.: H08-211790 also proposes changing the target voltage of the low voltage in the feedback control depending upon the output high voltage.
However, when the necessary amount of high voltage power increases to 300 W or greater due to certain circumstances (e.g., due to the inclusion, in an image forming device, of a sheet post-processing device that performs stapling of a stack of sheets with images formed thereon), guaranteeing accuracy of both the output low voltage and the output high voltage becomes difficult. Specifically, with the conventional technology disclosed in Japanese Patent Application Publication No.: H08-211790, accuracy of both the output low voltage and the output high voltage can no longer be guaranteed should the necessary amount of high voltage power become higher than around 200 W.
In view of this, a proposal is being made of a power supply device that includes an upper limiter circuit that limits output high voltage to be within ±10% of a rated voltage, while controlling output low voltage to be at a target voltage. Refer to Japanese Patent Application Publication No.: H09-093924 for one example of such a power supply device. However, this technology also has a drawback in that electric power loss occurs at the upper limiter circuit, and a consequent increase in power consumption occurs.